There are a variety of solid adsorbents which have been useful in adsorption and catalysis including commonly known materials such as activated carbons, activated clays, silica gel, activated alumina, and crystalline molecular sieves. Of these adsorbents, crystalline molecular sieves such as silicoalumino phosphates, aluminophosphates and aluminosilicate zeolites have been particularly useful because of their uniform pore size.
In many instances it is desirable to have the solid adsorbent deposited on a substrate as a coating instead of being contained in particulate form as pellets, beads, or other particles. There are several reasons why solid adsorbent coatings have been used including for example, to improve the catalytic or adsorption properties of the solid adsorbent by improving the surface area to weight ratio, to reduce the mount of solid adsorbent required, to protect the underlying substrate material from a harmful environment, to achieve a particular strength or form, and, to perform the particular adsorptive or catalytic function over the entire coated surface of the substrate.
Despite the diversity of coating methods and end uses known to exist, new methods are sought which can be used to coat the inside surfaces of tubes with solid adsorbents without the use of adsorbent formation reactions, frits and enamels, paints, varnishes and the like, in order to provide adsorbent-substrate composites that have physical and performance properties suitable for sorption cooling use.
Some thermodynamic processes for cooling and heating by adsorption of a refrigerating fluid on a solid adsorbent use zeolite, and other sorption materials such as activated carbon and silica gel. In these processes, the thermal energy from adsorbing zeolite in one place is used to heat desorbing zeolite located in another place. U.S. Pat. No. 4,138,850 relates to a system for such solar heat utilization employing a solid zeolite adsorbent mixed with a binder, pressed, and sintered into divider panels and hermetically sealed in containers. U.S. Pat. No. 4,637,218 relates to systems for a heat pump using zeolite as an adsorbent wherein the zeolite is prepared by slicing natural zeolite rock with a carbide saw, or by pressing slightly-wetted, powdered zeolite into bricks. The bricks used in U.S. Pat. No. 4,637,218 are preferably not more than 10 mm in thickness.
U.S. Pat. No. 4,548,046 relates to an apparatus for cooling or heating by adsorption of a refrigerating fluid on a solid adsorbent. The operations employ a plurality of tubes provided with parallel radial fins filled or covered with solid adsorbent such as Zeolite 13X located on the outside of the tubes.
The thermodynamic aspects of developing a zeolite-water adsorption refrigeration unit are well known. An article entitled, "Thermodynamic Analysis of a Solar Zeolite Refrigeration System," by S. Chang and J. A. Roux, which appeared in the Journal of Solar Energy Engineering, Aug. 1985, Volume 107, pages 189-194 provides a discussion of the main parameters, including adsorber properties.
In absorber/generator based cooling systems the most significant parameter is the overall heat transfer coefficient between the adsorbent bed and the cooling or heating gases per unit weight of adsorbent in the system. This parameter has been related in the literature to the cooling power per kilogram of adsorbent. The higher the cooling power, the more efficient the adsorber/generator system. Current systems are limited by requiring a high adsorbent regenerator temperature or a long cycle time to achieve relatively low cooling power values.
In a paper titled, "Application of Adsorption Cooling System to Automobiles," by Moloyuki Suzuki, presented at the Solid Sorption Refrigeration Symposium Paris, France, Nov. 18-20, 1992. Suzuki disclosed the results of a study to particularly point out the technological limits associated with the application of adsorption cooling systems to passenger car air conditioning. Suzuki's model considered an adsorbent bed wherein the adsorption step corresponds to the cooling step where water evaporation takes place at a water container, and wherein regeneration step corresponding to a generation step where the adsorbent bed is heated by exhaust gases to desorb the water. These steps are repeated in series requiring at least two units to achieve continuous cooling. Suzuki suggests the use of "quick cycles with a high overall heat transfer coefficient will result in acceptable designs. Currently, overall heat transfer coefficients in the ranges of 25 to 50 are reported in a paper title, "Reaction Beds for Dry Sorption Machines," by M. Groll and presented at the above mentioned Solid Sorption Refrigeration Symposium. Suzuki predicts a threshold value of 100 kW/m.sup.3 K (about 150 W/m.sup.2 K) for overall heat transfer k, m coefficient as a target for the future work, and further points out the need for systems with mechanical strength for use in automobiles, but does not suggest how this value which is greater than 3 times the ability of the current an can be achieved.
Prior methods of using zeolite adsorbents in devices for cooling or heating by adsorption of a refrigerating fluid on a solid adsorbent have been inefficient and difficult to prepare. Those methods of preparation included cutting natural rock into thin bricks and mounting these bricks on to heat exchange surfaces or casting powdered zeolites and mixtures thereof with clays into panels or slabs for direct contact with fluids. Methods are sought to improve the operating efficiency of these devices, and to improve the way in which the solid zeolite adsorbent is employed in these devices.